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Viola canina L., Heath Dog-violet

Account Summary

Native, rare, possibly under-recorded. Eurosiberian boreo-temperate.

1884; Barrington, R.M.; Devenish Island, Lower Lough Erne.

June to August.

Growth form and preferred habitats

V. canina is a ± hairless perennial with ovate-lanceolate, ± triangular-shaped, dark green leaves. The shiny leaves are strongly reticulate below. The flowers, produced from April to June, are a fine clear blue or bluish-grey colour, without a dark purple zone outside the whitish base of the lowest petal. They are furnished with a bright yellow, or more rarely greenish-yellow spur, usually deeply notched, but occasionally not. The vegetative plant has no stolons and no central leaf rosette, all leaves being borne on the flowering stems (Porter & Foley 2017; Sell & Murrell 2018). Green cleistogamous flowers or flower buds are produced in summer.

As its English common name suggests, Heath Dog-violet is chiefly found in unshaded acidic habitats and, apparently nowadays, it is mainly a plant of coastal distribution on more open areas of older, fixed, grey dunes and dune heaths across both B & I. However, the species does also occur scattered and much more rarely in open, vegetation gaps in inland situations including woods, stony heaths and grasslands over dry, shallow or sandy soils and in open, stony ground, or in crevices and ledges on rock outcrops. It can also occur on damp to moist, peaty ground near high water mark on stony riverbanks and on shingle around lakeshores. The waterside types of inland habitat are especially well represented in Scotland and Ireland, and the soils in them may be much more calcareous or base-rich than is the norm for this species elsewhere (Corner 1989; New Atlas).

In most or all of its sites, coastal and inland, the associated vegetation needs to be kept fairly open by periodic grazing to permit V. canina to thrive and flower. In its lake shore and riverbank habitats, competing plants are additionally limited by being subjected to occasional flooding after heavy or prolonged rainfall (Corner 1989; M.J.Y. Foley & M.S. Porter, in: Preston et al. 2002).

Variation

Two subspecies are recognised in Britain at least, if not confirmed for Ireland. Subsp. canina is the widespread form in B & I, while subsp. montana appears to be confined in these islands to a couple of fens in Cambridgeshire. The latter form has somewhat larger flowers than the more common type (15-22 mm long, as opposed to 7-18 mm in subsp. canina) and the stipules of the middle leaves of subsp. montana plants are less than half the length of the petiole (Sell & Murrell 2018).

Fermanagh occurrence

In Fermanagh, V. canina has been recorded from 14 tetrads (2.7%), but only seven of them have post-1975 dates. This suggests that Heath Dog-violet, which was always a very local species here, has declined during the last half century. The downward trend also appears elsewhere in B & I during the same period, and losses of this widespread but rather thinly scattered violet appear particularly obvious at its previous inland sites across these islands (M.Y.J. Foley & M.S. Porter, in: Preston et al. 2002). Nevertheless, RHN and I believe V. canina is probably under-recorded in Fermanagh, at least to some extent, perhaps on occasions being mistaken for V. riviniana (Common Dog-violet) (Rich & Woodruff 1992). We suggest that modern botanists, including ourselves, have not sufficiently searched likely inland habitats for this violet, whose flowers are a fine clear blue with a whitish or pale yellow spur.

Irish turlough occurrences

One of the most typical habitats of V. canina in Ireland is in an open, well illuminated zone on the upper shore levels of turloughs or vanishing lakes. These grassy or rocky hollows in limestone areas are intermittently flooded after heavy rainfall and they drain vertically into cave systems (Praeger 1932). Almost a third of the Fermanagh records of V. canina (five of the 17) are from three turlough sites in the Ely Lodge area, NW of Enniskillen. Unfortunately, in the last ten or 15 years, despite their listing as ASSI conservation sites, these limestone hollows have become overgrown by taller,

herbs and dense, turf-forming, strongly competitive graminoid species, so that active survival of a shade intolerant violet like V. canina has become improbable or impossible. However, V. canina seed can persist in the soil for many years (Thompson et al. 1997) and the plant populations may therefore recover at a later date if the ground is cleared sufficiently and the vegetation reopened. More active management is required to conserve the floral biodiversity of the unique turlough habitat.

Ecological overlap and hybrids

V. canina typically occupies an open, well illuminated zone on turlough shores about a metre or so above the much rarer and more ecologically restricted V. persicifolia (Fen Violet), or in Ireland, 'Turlough Violet', which, as the latter name indicates, is another characteristic species of this unique habitat. In the intermediate zone where these two violets meet, hybrids are sometimes found in some abundance (Praeger 1932; Webb & Scannell 1983). Hybrids are also quite commonly found when V. canina and V. riviniana meet in grasslands or heaths.

Other local sites: Elsewhere in Fermanagh, apart from the turloughs, V. canina has been recorded, although very rarely, on the shingle or rocky shores of most of the larger lowland lakes. Praeger's 1900 record from the village of Garrison (no habitat details given) is possibly from the rocky or stony riverbank. However, even this record (mention of which appears only in his monumental book, familiarly referred to by Irish field botanists by the initials ITB (Praeger 1901c)), might well originate from the nearby shore of Lough Melvin, since Praeger rowed to the Trollius europaeus (Globeflower) site on the shore of the lough from Garrison on 27 June 1900, after an overnight stay in the village (Praeger 1901a).

Fossil record

Although the genus Viola is not well represented in the fossil pollen record for obvious reasons, its seeds are commonly met in such deposits, although distinguishing the individual species is another matter. Godwin (1975) lists records for V. canina from glacial deposits at Kirkmichael, the Isle of Man dated to 10,000 BP (zone I/II transition and zone II) and there are tentative earlier records from the Lea Valley arctic beds in Essex, plus finds from Bronze and Iron age sites in Britain. The small, scattered and, now with the prospect of global warming, increasingly vulnerable populations of this violet are thus very probably relicts of long past, possibly early post-glacial, more widespread open habitats with unleached, nutrient-rich soil conditions (Pigott & Walters 1954). These populations have managed to survive locally in pockets of specialized, relatively uncompetitive growing conditions, despite massive long-term changes in climate, vegetation and soils (Corner 1989).

Clearly this is a species worthy of conservation. The current population status and behaviour, with respect to land use in its inland sites in our area, is worthy of further more intensive investigation, as indeed is also the case elsewhere in these islands, at least in inland sites.

European occurrence

The wider distribution of V. canina subsp. canina extends over much of boreal and temperate Europe stretching from Iceland and N Fennoscandia eastwards to middle latitudes in Siberia and southwards into the northern half of Italy, Corsica, Sardinia and extending eastwards at these latitudes to the shores of the Caspian Sea (Hultén & Fries 1986, Map 1327).

Threats

The Fermanagh turloughs are now ASSIs, as is much of the shore of Upper Lough Erne, but in recent years disputes with landowners and lack of adequate grazing regimes have resulted in several of these important sites becoming rather overgrown with taller vegetation. I am seriously concerned that rare or relatively scarce species like V. canina and V. persicifolia will be, or have already been, ousted by more vigorous competitors.